Fuel cells are devices in which a fuel, such as methanol, ethanol, hydrogen or corresponding mixtures, may be burned in a controlled manner using an oxidizing agent, such as pure oxygen, air, chlorine or bromine gas, the reaction energy released in the process being converted into electrical energy. Such fuel cells have been used to generate electrical energy for several decades. Due to their high efficiency, their low or completely absent harmful substance emission, and their low noise development during operation, the interest in the use of fuel cells has risen drastically in many fields in recent years. In particular the vehicle and power plant fields shall be mentioned in this regard.
Fuel cells are typically classified according to the type of the electrolyte which separates the anode and cathode chambers from each other. A particularly interesting fuel cell type, which is suitable in particular for use in smaller power plants and for mobile use (as an energy source for the electric motor vehicle drive, for example), is the polymer electrolyte fuel cell. This type of fuel cell uses an ion-conducting membrane as the electrolyte. A single solid polymer fuel cell generally includes a so-called membrane electrode assembly (MEA), in which an ion-conducting membrane is situated between a cathode and an anode. The ion-conducting membrane simultaneously serves as a partition and as the electrolyte. Catalyst particles, which promote the conversion reactions in the fuel cell, are situated at the boundary between the electrodes and the membrane. The electrodes are typically in contact with porous current collectors, which additionally stabilize the electrode structure and allow fuels to be supplied. Since the operating voltage of a single cell is normally less than 1 volt, most fuel cells are made up of a cell stack, in which numerous individual cells stacked on top of each other are connected in series to generate a higher voltage.
Since the electrochemical reaction between the fuels takes place exothermically, the fuel cell usually must be cooled so that the desired operating temperature may be maintained and damage to the membrane may be avoided. Since a relatively large amount of heat having only a small temperature difference compared to the ambient temperature must be dissipated, typically liquid coolants are used which have a sufficiently high heat capacity. For this reason, aqueous coolants are particularly well-suited. In general, mixtures of water and ethylene glycol are used as antifreeze agents, as they are known for cooling internal combustion engines. To prevent metallic components of the cooling circuit and of the fuel cell from corroding, the coolants in general additionally contain non-ionic corrosion inhibitors.
A special characteristic of fuel cell cooling is the requirement of a very low electrical conductivity of the coolant to counteract the risk of electrical short circuits between the individual cells of the fuel cell stack. A coolant made of deionized water, glycol, non-ionic corrosion inhibitors and other additives is used for this purpose.
If deionized water is used as the coolant, this may simultaneously be used to humidify the reactants flowing into the fuel cell in order to ensure sufficient humidification of the polymer membrane. Depending on operating conditions, it may be necessary to add an antifreeze agent, such as ethylene glycol, or other additives, to the cooling water. However, the materials installed in the cooling system and in the fuel cell introduce ions into the coolant, which increase its electrical conductivity. To counteract this effect, deionization devices having ion exchange resins are used, around which the coolant flows. The ion exchange resins take up ions (cations and anions) dissolved in the coolant and give off H+ and OH− ions, which combine again to form H2O.
Ion exchangers for cooling media of fuel cells are known, for example, in granular form in ion exchanger cartridges from DE 10 2009 0370 080 A1, DE 10 2011 009 923 A1, and DE 10 2011 009 917 A1. Moreover, a liquid deionizing agent is proposed in DE 102 01 276 A1, which is admixed to the coolant and is separated from the coolant after the ion exchange with the aid of a phase separator.